Tethered catalysts have been used in asymmetric transfer hydrogenation reactions (see, for example, Hayes et al, J. Am. Chem. Soc., 2005, 127, 7318, Cheung et al, Organic Letters, 2007, 9(22), 4659, Morris et al, J. Org. Chem., 2006, 71, 7035 and Martins et al, J. Organomet. Chem., 2008, 693, 3527). The transfer hydrogenation conditions utilise formic acid and triethylamine. A hydrogenation reaction differs from a transfer hydrogenation reaction in that hydrogen gas is used and not reagents such as formic acid and triethylamine.